1. Field of the Invention
The invention relates to an automatic two-speed transmission (i.e. a two-speed gear unit which on reaching a predetermined speed of rotation automatically changes the transmission ratio) for the drive of one or more auxiliary units (e.g. the dynamo, the fan, a refrigeration compressor or a power steering pump) of a motor vehicle internal-combustion engine (usually this is a reciprocating-piston engine but it can also be for example a rotary piston engine) comprising a gear set in which at least one gear of which the axis of rotation orbits about the axis of rotation of another gear concentric with the input shaft of the transmission is in engagement with said other gear, a control element which is formed as freewheel, and a further control element rotating with the input shaft and actuable by centrifugal force for retarding and releasing the relative rotation between the gears of the gear set.
2. Description of the Prior Art
Motor vehicle internal-combustion engines operate in a very large speed of rotation range. If certain auxiliary units are driven with a constant transmission ratio by the engine then at higher speeds of rotation they are driven with a far higher speed than necessary because at the lowest speed of rotation, i.e. the idling speed of the engine, these auxiliary units must also be driven at a certain minimum speed. A result of this is that auxiliary units at higher speeds of rotation take up far more power than is necessary. To overcome this difficulty automatic two-speed gear units or transmissions of the aforementioned type have been proposed. They ensure that when a certain speed of rotation is exceeded the transmission ratio with which the corresponding auxiliary units are driven is reduced and when said speed is exceeded the reduced transmission ratio is again increased to the previous value. Basically of course an infinitely variable transmission would be ideal for this purpose. However, such a transmission is too complicated in production, too heavy and too large for installation in a motor vehicle.
Consequently, a two-speed transmission is accepted and can be made relatively small, light and with relatively few components. Such an automatic two-speed or two-step transmission is known in which as is also preferred in the invention the input shaft is the crankshaft of the engine or an extension thereof. Non-rotatably connected to the crankshaft is the ring gear of a planetary transmission or gear unit. The input shaft is surrounded by a sleeve which is non-rotatably mounted on the engine housing and on which the sun gear of the planetary transmission is mounted by means of a freewheel. The planet gear carrier of the planetary transmission is in turn rotatably mounted on said sleeve and carries at the outside the output means such as a V-belt groove. The planet carrier can be blocked with the sun gear by means of a mechanically engageable and disengageable centrifugal force clutch or coupling At low speed of rotation of the engine the multiple disc clutch between the planet carrier and the sun gear is closed so that the V-belt pulley serving as output revolves with the engine speed. On reaching a predetermined speed of rotation the centrifugal force clutch opens so that the planet carrier can now revolve relatively to the sun gear. The sun gear rigidly connected to the input shaft now drives via the planet wheels the planet carrier so that the latter revolves with reduced speed. When the speed of rotataion drops this process takes place in the reverse direction.
In transmissions of the type improved with the invention it must of course be ensured that the changing up of the transmission ratio when the speed of rotation drops takes place at a lower speed of rotation than the changing down of the transmission ratio with increasing speed because otherwise the clutch would continuously change up and down within a very narrow speed range. A certain hysteresis effect is therefore necessary. In the known transmission this is to be achieved by the friction in the centrifugal force regulator.
Transmissions of this type, from which the invention proceeds, and other known similar transmissions or gear units have substantial disadvantages. One of them is the mechanical shift mechanism which is prone to wear. This is extremely disadvantageous because transmissions of the type in discussion change speed very often, for example far more than an automatic transmission in a motor vehicle drive train. A further defect is the large axial extent which is usually required and which should be kept as short as possible in particular on installation of the engine with the crankshaft in the travelling direction. Furthermore, many of the known transmissions are relatively complicated in construction.